We propose to study the replication of one of the "non-defective" Parvoviruses, the Minute Virus of Mice (MVM). The Parvoviruses are extremely small and contain a single-stranded DNA genome of l-2 x 10 to the 6th power daltons, among the smallest viral genomes presently known. These viruses have a high selective affinity for certain types of growing cells, particularly tumor cells. In addition, they maintain some form of a "latent" state in infected adult animals. The objectives of our experiments are to identify and characterize the cellular mechanisms which restrict MVM replication to the S phase of the host cell growth cycle. Specifically, we propose: 1) To determine if specific steps of viral replication occur in non-growing cells including uncoating, conversion of the parental single-stranded DNA to a double-stranded form and transcription. Such experiments are designed to indicate whether the latent state observed with these viruses occurs at the cellular level with the virus establishing a quiescent relationship with non-growing cells. 2) To isolate intact and biologically active viral transcripts from lytically infected growing cells, to characterize these molecules, and to compare them with any viral RNA produced in non-growing cells. A study of viral transcription will indicate possible points of control of viral expression by the infected cell.